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    <title>Document</title>
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<body>
    <script>
        const arr = [1,[2,[3,[4,5]]],6]
        console.log(arr);
        console.log(myFlat(arr));
        function myFlat(arr,res=[]){
            
            if(arr.length==0){
                return;
            }
            for (const iterator of arr) {
                if(Array.isArray(iterator)){
                    myFlat(iterator,res)
                }else{
                    res.push(iterator)
                }
            }
            return res;
        }

        function myFlat(arr){
            
           let string=JSON.stringify(arr);

           string='['+string.replace(/\[|\]/g,'')+']'

           console.log(string)
           return JSON.parse(string)
            
        }


    </script>
</body>
</html>